HDV superinfection of chronic hepatitis B leads to rapid progression of liver cirrhosis, but the mechanisms that drive disease pathogenesis are not known. HDV-specific T cells have not been detected in chronic patients and only very few CD8 T cell epitopes were identified in rare recovered patients. Reasoning that a surge in viremia after lonafarnib/ritonavir antiviral therapy may boost T cell responses and facilitate epitope mapping in chronic HDV infection, we established HDV-specific T cell lines from post-treatment bleeds of chronic HDV patients and mapped CD8 T cell epitopes with pools of overlapping HDV peptides and dose titrations of 15-mer and shorter peptides. IFN-gamma production was used as read-out. With this approach, we identified nine CD8 T cell epitopes. These were used to synthesize MHC-epitope multimers and to characterize HDV-specific CD8 T cells ex vivo in untreated patients. The frequency of HDV-specific CD8 T cells was similar to that of HBV-specific CD8 T cells in HDV/HBV co-infected patients, but lower than the frequency of CMV, EBV and Flu-specific CD8 T cells. Most virus-specific CD8 T cells were effector memory cells (CCR7+CD45RO+). About 40% of HDV- and EBV-specific CD8 T cells were activated (CD38+) as compared to about 20% of HBV-specific CD8 T cells and <10% of Flu-specific CD8 T cells, and the percentage of activated cells correlated with liver transaminases. IFN-gamma production was detected upon ex vivo peptide stimulation. Viral sequence variations and stop codons were found in most epitope-encoding sequences. Ex vivo IFN-gamma production and viral escape mutations indicate that HDV-specific CD8 T cells are functional in vivo.